1. Field of the Invention
The present invention relates to a method and a system for quantum key delivery, and more particularly to such a system and a method for use in quantum cryptographic telecommunications system.
2. Description of the Background Art
It is anticipated that quantum cryptographic communication systems will achieve ultimate secrete communications and be applied to future, high-security information communication systems.
In order to realize a quantum cryptographic telecommunications system, either a single photon source for generating a single photon per pulse or a photon entanglement source for generating a pair of quantum entangled photons per pulse is necessary. From a viewpoint of distant telecommunications, telecommunications systems using quantum entangled photon pairs are promising in the future.
One of the solutions used heretofore as means for generating pairs of quantum entangled photons is to use spontaneous parametric fluorescent light in a second- or third-order nonlinear optical medium. More specifically, when a second- or third-order nonlinear optical medium receives exciting photons having a wavelength of λp, a wave number of kp, and an optical angular frequency of ωp, it produces a signal photon having a wavelength of λs, a wave number of ks and an optical angular frequency of ωs as well as an idler photon having a wavelength of λi, a wave number of ki and an optical angular frequency of ωi. This is a process in which spontaneous parametric fluorescence occurs. At this time, a signal photon and an idler photon are simultaneously produced necessarily in pair.
If a second-order nonlinear optical medium is used, relationships given by the following Expressions (1) and (2) are established between the wave numbers and optical angular frequencies of the exciting photon, signal photon and idler photon to correspond to the law of conservation of momentum and the law of conservation of energy, respectively.kp=ks+ki+K  (1)ωp=ωs+ωi  (2)When a second-order nonlinear optical medium is used, spontaneous parametric fluorescence is also known as spontaneous parametric downconversion (SPDC).
If a third-order nonlinear optical medium is used, spontaneous parametric fluorescence is also known as spontaneous four-wave mixing (SFWM). The wave number and optical angular frequency of each photon satisfy relationships given by the following Expressions (3) and (4).2kp=ks+ki+K  (3)2ωp=ωs+ωi,  (4)where K contained in Expressions (1) and (3) is a parameter corresponding to the period of a periodically modulated structure of nonlinear optical coefficients. Today, such periodically modulated structures of nonlinear optical coefficients are frequently used in, e.g. cases where a crystal of LiNbO3, described later, is adopted as a nonlinear optical medium with the purpose of enhancing the nonlinear optical effect by pseudo phase matching.
Besides the correlative relationships of wave number and optical angular frequency described above, there is a correlative relationship in polarization between a signal photon and an idler photon. Pairs of signal and idler photons produced in spontaneous parametric fluorescence and correlated with each other in that way, i.e. quantum entangled photon pairs, are generally termed quantum correlated photon pairs or simply correlated photon pairs, which may be hereinafter referred to.
Apparatus capable of implementing a method of making use of such correlated photon pairs to obtain quantum entangled photon pairs is a quantum correlated-photon pair generator. A quantum key delivery system for accomplishing a quantum encryptic telecom system that realizes communications over long distances such as 100 km to 200 km using a quantum correlated-photon pair generator has been heretofore reported by H. C. Lim, et al., “Stable source of high quality telecom-band polarization-entangled photon-pairs based on a single, pulse-pumped, short PPLN waveguide”, OPTICS EXPRESS, Vol. 16, No. 17, pp. 12460-12468 (2008), and J. F. Dynes, et al., “Efficient entanglement distribution over 200 kilometers”, OPTICS EXPRESS, Vol. 17, No. 14, pp. 11440-11449 (2009).
However, there still exist many problems in order to accomplish a quantum key delivery system permitting long distance communications over tens of km or more and having practical stability.
Firstly, practical devices or system for attaining a practical quantumkey delivery system needs to continue to stably operate over a long term while at least maintaining predetermined values. For example, a performance required for a light source applicable in currently predominant optical communication systems is the capability of stably maintaining the optical output value over a long term.
However, practical devices or system, when continuously used under the same conditions, vary in characteristics with age. Therefore, practical devices or system is required to be equipped with mechanisms which can detect whether or not the practical devices or system is out of the prescribed operational state and which permit a deviation thus detected to be fed back so as to restore the intended prescribed state thereof.
In the existing optical communication systems, semiconductor lasers or the like are used as its light source, and are attempted in stabilizing the output value thereof by the following measures. Namely, part of the output light from a light source, e.g. laser, is branched off to monitor its intensity so that, if the intensity is above or below a prescribed value, the driving current is increased or reduced accordingly to thereby stabilize the output light from the light source. Alternatively, the resonator has its one end surface serving as taking out an output light and its other end surface serving as monitoring the intensity of the output light so that, if the intensity is above or below a prescribed value, the driving current can be increased or reduced accordingly to thereby stabilize the output light of the semiconductor laser.
In view of the foregoing, it is desired that, in a practical quantum correlated-photon pair generator capable of being used in actual information communication systems, at least the number of correlated photon pairs contained per pulse be stabilized at a given average, or expected value over a long period of time, and that some solution or other be established for ascertaining that the number remains stabilized.
On the other hand, quantum key delivery systems are operable on the assumption that one photon is used per pulse, i.e. a pair of photons is used per pulse. If plural photon pairs are present per pulse, it would be possible to take out some of the pairs for eavesdropping. That would greatly impair the secrecy provided by quantum encryption.
Accordingly, it is impossible to apply the solution used in the aforementioned existing light sources for optical communications to quantum correlated-photon pair generators in order to stabilize the light source output because the branching-off and detection of some of the photons would inherently annihilate photons conveying signals, and because the measurement does affect or destroy the quantum state, making it impossible to transmit correct information to the recipient.
In particular, there has been needed a solution for assuring that a quantum correlated-photon pair generator stably operates while maintaining its preset state, e.g. continuously stably generates a single pair of correlated photons. However, there is yet no report on how to implement such a solution.
Now, secondly, in a telecommunications system having a transmission distance longer than tens of kilometers, the temperature dependency or the like of the refractive index of the free space or optical fiber forming a transmission path causes signals to arrive at random moments. Therefore, in order to accurately receive signals in a long-distance telecom system, a clock extraction function for extracting a clock signal from the incoming signals is indispensable. That is also the case with quantum information communication systems.
Thus, when implementing a quantum key delivery system, in order to accomplish a long-distance key delivery system stably operating over a long term, it is desired to accomplish the function of monitoring whether or not an expected value of the number of correlated photon pairs is maintained substantially constant over a long period of time and a clock extraction function of extracting a clock signal for detecting the arrival of photons.